In projectors based on a so-called “flying spot” functional principle, by means of a two-dimensional resonant micromirror light beams (typically consisting of the three primary colors, red, green and blue), are deflected and projected onto an image plane.
FIG. 1 shows a schematic diagram for illustrating the functional principle of “flying spot” projection. In this case, light beams of different colors from laser sources 101 (red R), 102 (blue B) and 103 (green G) are respectively directed onto a semitransparent mirror (the transmission and the reflection of the mirrors are effected in a manner dependent on the wavelength), 104, 105, 106 and are then directed as a common beam 110 (also referred to as projection beam) onto a two-dimensional resonant micromirror 107, which deflects the common beam 110 two-dimensionally and projects it onto an image plane 108. In this case, in the image plane 108, the image is built up by the continuously harmonically deflected common beam 110 (see beam profile 109 in the image plane 108).
An image information item is generated and represented by means of an intensity modulation of the respective light source 101, 103 synchronously with the deflection of the micromirror 107.
On account of the nonlinear deflection of the micromirror 107 and the resultant nonlinear beam profile 109 in the image plane 108, a time division multiplex method is used for representing individual, locally discrete image information items (“pixels”): consequently, in defined time segments, specific information items are projected onto the image plane.
Projected information means, in particular, a superimposition of the brightnesses and colors of the light beams generated by the light sources 101 to 103, it being possible for the brightness of a light beam to be set on the basis of an amplitude of the associated light source.
Preferably, the light sources are in each case a laser, in particular a laser diode. Consequently, the current through the laser corresponds to the brightness of the light emitted by said laser.
FIG. 2 shows an illustration of scan time ranges per pixel in seconds as a function of a position of the respective pixel, to be precise both for a column (see curve 201) and for a line (see curve 202).
By way of example, a projected image has a width of 640 pixels and a height of 480 pixels. The deflection 109 of the common beam 110 as described and shown in FIG. 1 has the effect that, in the case where the image is built up line-by-line, for example, the common beam 110 is significantly faster in the center of the line than in an edge region.
By way of example, the micromirror in the example in accordance with FIG. 2 has a horizontal frequency of 27 kHz and a vertical frequency of 1.2 kHz with a resolution of 640 times 480 pixels.
A time range, a temporally governed length and also a duration for each pixel thus result from a spatial assignment of the pixels in an XY coordinate system over time by means of a time division multiplex method.
FIG. 3 shows an excerpt from FIG. 2 for the time ranges along a (horizontal) line in the region of the center of the image plane (image center).
On the basis of the parameters mentioned above it becomes clear that the required temporal resolution of the electronics for a modulation of the intensity or amplitude of the light beams for the locally error-free representation of the image information on the projection area lies in a range which is smaller than one picosecond. An assignment error could theoretically be reduced with complex circuits having a correspondingly high temporal resolution. However, such complexity requires expensive components and is not always feasible in practice, e.g. depending on the resolution chosen.
If the temporal resolution is reduced, however, the image quality is reduced and distortion occurs at the pixel level on account of the absent spatial assignments.
A further problem consists in the transformation between time domain and space domain on account of the nonlinear deflection of the micromirror.
In a time segment in which a pixel is selected by the time division multiplex method, rise and fall times of the electronics influence the contrast between the pixels. This influence is intensified by the duration of a rise and/or fall of a signal edge: the longer the edge in relation to the time period available for the pixel, the poorer the contrast between the pixels.
In accordance with the above explanations concerning FIG. 2 and FIG. 3, given constant edge steepness, the contrast is the poorest when time is available the least overall for the pixels, that is to say in the image center.
FIG. 4 shows by way of example an excerpt from an image to be projected in an image plane with the greatest possible contrast, that is to say a transition from white to black or vice versa between two respective pixels.
FIG. 5 shows in a correspondingly simplified manner a drive voltage 501 for a laser and a resultant current 502 through the laser.
The current profile 502 through the laser is typically proportional to an emitted quantity of light and thus corresponds to the brightness perceived by an observer.
FIG. 5 illustrates the pixels n−2, n−1 and n, which have a time duration Tp, for example, and which have a bright-dark-bright pattern in accordance with FIG. 4. The drive voltage 501 turns the laser on, off and on again.
On account of the finite edge steepness, the switch-on delays 503 and 505 arise in the course of the laser being switched on and switch-off delays 504 and 506 arise in the course of the laser being switched off.
These delays significantly vitiate the contrast between the pixels. In particular, the dark pixels are partly illuminated during the delays 504 and 506, as a result of which a maximum attainable contrast of the projection unit during the representation of edges in the image (high spatial frequency) decreases significantly.
FIG. 6 shows a block diagram for the driving of a laser 603.
A digital signal 605 having a width of n-bits is converted into an analog signal by a digital/analog converter 601 (DAC) and amplified by means of a driver for the driving of the laser 603. The laser 603 is connected by its anode to a supply voltage 604 (VDD) and is driven via the digital/analog converter 601 by means of a driver 602.
An image source, e.g. a personal computer or a personal digital assistant (PDA) supply digital signals 605 having a width of n-bits, which are correspondingly converted by the digital/analog converter 601 into an analog signal (current signal or voltage signal) for the driving of the laser 603.
The high temporal resolution required is attained by means of a high conversion rate (sampling rate). This imposes extreme demands on the conversion rate of the digital/analog converter 601.
If, by way of example, the above-described system having a resolution of 640 times 480 pixels, a horizontal frequency of 27 kHz and a vertical frequency of 1.18 kHz is considered, then the digital/analog converter 601 has to provide a temporal resolution of less than 280 ps in order that the local resolution error lies below 1%.
This means a bandwidth demand of 3 GHz imposed both on the digital interfaces and on the conversion rate of the digital/analog converter 601.
Such electronics, if they can be realized at all, are therefore extremely complex, lossy and expensive. Furthermore, it should be taken into account that the circuit illustrated in FIG. 6 and the resultant complexity become necessary separately for each laser and the costs associated therewith multiply.